1. Field of the Invention
The invention relates to a method of measuring echo delay, used in an echo-cancelling arrangement incorporated in a transceiver arrangement to cancel an echo signal occurring in the receive path in response to a signal supplied to the transmit path and consisting of a substantially undelayed near echo and a delayed distant echo, this echo cancelling arrangement comprising a near echo canceller receiving a signal from the transmit path and a distant echo canceller receiving said signal from the transmit path with a delay substantially equal to the measured distant echo delay.
2. Description of the Prior Art
It is known that echo cancellers are adaptive devices which are incorporated in, for example, data transmission modems connected to a two-way transmission circuit in order to cancel automatically undesirable echoes occurring in the one-way receive path of the modem in response to the signal supplied to the one-way transmit path of the modem. Conventional echo cancellers are generally designed to cancel echo signals which are not delayed or relatively little delayed in relation to the transmitted signal and which may occur during a time interval of 40-50 ms following the instant of originating the transmitted signal. In practice, these characteristics are sufficient to cancel the echoes occurring on all national and international terrestrial circuits.
However, international communications are being conducted more and more via communication satellites. In a circuit of this kind, including a satellite link between two radio-relay stations, it is possible that there will occur in the receive path of a modem a echo which is not or little delayed and is generated in the part of the circuit preceding the satellite link, as well as a distant echo which is generated in the part of the circuit after the satellite link and which is therefore subject to a considerable delay .tau., depending particularly on the wave-propagation time in the satellite link. Depending on whether the satellite is geostationary or not and on the variation in the terrestrial link, it can be estimated that in the international switched network the delay .tau. of the distant echo may vary between approximately 220 and 630 ms.
To cancel simultaneously the near echo and the distant echo, which each have a relatively short duration of the order of 10 ms or several tens of ms but which are separated by a large time interval of the order of the delay .tau., it is an advantage to use the configuration described above and known from the article by Stephen B. Weinstein, entitled "A Passband Data Driven Echo Canceller for Full-Duplex Transmission on Two-Wire Circuits" and published in the journal IEEE Transactions, Vol. COM-25, No. 7, July 1977, pp. 654-666. This configuration comprises a section for cancelling the near echo, a delay line simulating the delay .tau. and connected thereto a section for cancelling the distant echo. This configuration makes it possible to use two conventional echo cancellers of reasonable complexity, but necessitates measurement of the delay .tau. of the distant echo in order to introduce this information into the delay line. In the above-mentioned article it is proposed, in order to measure this delay, to apply to the transmit path a pulse formed by a 1000 Hz sinusoidal signal lasting several ms and to determine the resulting distant echo delay with a moving-window power detector. With this method, the signal used to measure the echo delay is therefore only transmitted for a short time, which is a disadvantage in satellite communications for which the transmission of energy has to be permanent in a communication path in order to maintain that path.